This invention relates to a variable capacitance pressure transducer of the type which utilizes a plate of electrically conductive semi-conductor material, such as silicon, as a diaphragm material. This conductive plate is sandwiched between a pair of plates of inorganic electrically insulating material, such as borosilicate glass, with the plates being electrostatically bonded together to form the transducer. Silicon-diaphragm transducers of this construction typically have a pair of concentric circular recesses etched in the opposite faces of the silicon plate or, alternatively, in the surfaces of the glass plates so as to form cavities on opposing sides of the silicon plate to define a diaphragm area on that plate. Circular, thin-film metallic deposits are laid on the surface of the glass plates in the cavities so that the deposits form capacitors with the diaphragm.
When the cavities are exposed to different pressures, the diaphragm deflects and increases the electrical capacity on one side while decreasing the electrical capacity on the other side. Thus, it is possible with a measuring circuit responsive to those changes in electrical capacity to obtain a pressure difference measurement. This measurement may be a measure of gage pressure when one cavity is exposed to the ambient pressure or it may be what is termed a pressure difference measurement when cavities on opposite sides of the diaphragm are both exposed to process pressures.
Pressure transducers such as described above are shown in U. S. Pat. No. 4,257,274-Shimada et al, issued on Mar. 24, 1981. A method for producing pressure transducers of this type is described in U.S. Pat. No. 4,261,086 issued to Giachino on Apr. 14, 1981.
The pressure range of pressure differences which can be measured by devices of the type described above is determined by the diameter of the diaphragm area and the diaphragm thickness. For a fixed diameter of the diaphragm area, the range can be increased by increasing the thickness of the diaphragm. As the range increases, the forces acting on the diaphragm increase in proportion to the pressure times the diaphragm area. A pressure will be reached where those forces exceed the material strength. In order to avoid failure, the choice of dimensions for the cavity and the diaphragm will normally be made by specifying first a cavity diameter small enough to contain the pressure being measured and second, the thickness of the diaphragm is chosen to give the desired diaphragm deflections for the pressure range being measured. It is, of course, desirable to extend the pressure range of these devices as much as possible to provide units which are broadly applicable.
The inventors have noticed that upon exposure of pressure transducers of this type to excessive pressure differences, failure of the transducer occurs as a result of excessive diaphragm deflections, which causes radial cracking of the glass plate on the high pressure side. It is therefore an object of the present invention to prevent the formation of such cracks in the pressure range in which they normally develop and thereby extend the pressure range of this type of transducer.